1. Field of the Invention
The present invention concerns a method for acquisition of measurement data by magnetic resonance (in the following, the abbreviation MR stands for magnetic resonance) as used in particular in the acquisition of measurement data in which a fat signal is suppressed and/or in which a contrast agent is injected. Furthermore, the invention concerns a magnetic resonance apparatus for implementation of such a method.
2. Description of the Prior Art
MR technology is a technology known for some decades with which images of the inside of an examination subject can be generated. Described with significant simplification, for this purpose the examination subject is positioned in a strong, static, homogeneous basic magnetic field (field strengths of 0.2 Tesla to 7 Tesla and more) in an MR apparatus so that the nuclear spins in the subject orient along the basic magnetic field. To trigger magnetic resonances, radio-frequency excitation pulses are radiated into the examination subject, and the triggered nuclear magnetic resonances are measured (detected) and MR images are reconstructed based on these signals. For spatial coding of the measurement data, rapidly switched gradient fields are superimposed on the basic magnetic field. The acquired measurement data are digitized and stored in a k-space matrix as complex numerical values. An MR image can be reconstructed from the k-space matrix populated with values by means of a multi-dimensional Fourier transformation.
MR technology is characterized by a variably adjustable tissue contrast that can be achieved by adapting the radiated magnetic fields and the excitation pulses.
A fat signal is often suppressed to achieve a desired tissue contrast. That term “fat signal”, means a signal that is generated by fatty tissue protons. A suppression of this signal can be achieved, for example, by radiating an inversion pulse is radiated, or by radiating a saturation pulse that is frequency-selective for fatty tissue protons at a temporally defined interval before the scanning of k-space (i.e. before the acquisition of the actual measurement data). However, this technique can significantly lengthen the measurement duration of an MR examination. Therefore multiple readout modules are often executed after a pulse for fat suppression, such that multiple k-space lines are scanned after this pulse.
In this technique, k-space to be scanned is sub-divided into multiple segments. A k-space line from a segment is scanned with each readout module after a pulse. Given three-dimensional measurements, k-space can be divided into radially arranged segments, for example, as is shown in FIG. 2. FIG. 2 shows a plane of three-dimensional k-space 40, wherein k-space lines 41 run perpendicular to this plane. The segments 42, 42′, 42″, 42′″ are typically read out in a central scanning scheme after a pulse for fat suppression. This means that a k-space line 41 of the central segment 42 is scanned first, followed by a k-space line 41 of the next segment 42′ etc. This continues until a k-space line 41 of the outermost segment 42′″ has been scanned. A pulse for fat suppression is subsequently radiated again. Different k-space lines of the segments 42, 42′, 42″, 42′″ are scanned this time in a similar manner, beginning at the central segment 42. This procedure is continued until a desired number of k-space lines 41 have been scanned.
Such a scanning scheme has the advantage that artifacts that are caused by movements of the subject to be examined (for example due to breathing movements) are smeared in the image and do not generate interfering structures in the image. Moreover, the quality of the fat suppression can be decoupled from the spatial resolution with such a scanning scheme.
Nevertheless, the need exists to improve such sequences.